For household electrical appliances and building materials, a hot-dip plated steel sheet with a plating composition containing Zn, Al, and Mg as main components, to which, if necessary, an addition element such as Si is added, has been broadly used or proposed. Such plating exhibits superior corrosion resistance compared to Zn plating or Al plating owing to an alloy element.
In recent years, innovations, such as control of a particle size by optimization of a bath composition, control of a particle size by a cooling method, etc., control of existence form through different pretreatments for plating, and rationalization of a phase by regulation of a cooling method, have been added in order to improve corrosion resistance or consistent appearance
For example, the following Patent Literature 1 discloses an invention concerning a hot-dip Zn—A—Mg coated steel material, in which plated layer one or two single phases out of MgZn2, or Mg2Zn11 are precipitated in a particle size of 0.5 μm or more, and which is superior in corrosion resistance at an unpainted processed part and a painted edge.
This invention has made clear the existence form of Mg superior in corrosion resistance. Specifically, it demonstrates that for improvement of corrosion resistance Mg should preferably be present independently in a plated layer forming a single phase of MgZn2 or Mg2Zn11 in a particle size of 0.5 μm or more, rather than distributed minutely in a ternary eutectic in a form of an intermetallic compound.
The following Patent Literature 2 discloses an invention concerning a hot-dip zinc-coated steel sheet, in which plated layer one or two out of an Al phase, a Zn phase, MgZn2, or Mg2Zn11 in a form of granular crystallites with a size of 0.3 μm or less are dispersed, and which is superior in corrosion resistance after working.
This invention alleges that working cracks can be decreased and the corrosion resistance after working can be superior because a constitution in which crystallites of the phase and the compound with an average particle diameter of 0.3 μm or less is dispersed randomly is formed due to a cooling condition of from 40 to 100° C./sec.
The following Patent Literature 3 discloses an invention concerning a Zn—A—Mg coated steel sheet having a plated layer on at least one side of the steel sheet, wherein a Mg—Zn compound contained in the plated layer does not exist as an aggregate, and grows from the vicinity of an interface between the plated layer and a steel matrix toward the superficial layer of the plated layer in a columnar shape to exist in a columnar shape exposing to the surface of the plated layer such that the exposed area occupies from 15 to 60% of the surface of the plated layer.
This invention alleges that by growing a Mg—Zn compound in a columnar shape by performing Ni plating, etc. as a pretreatment, so as to dissolve the Mg—Zn compound gradually at a constant rate from the initial stage of corrosion until complete depletion of the whole plating, an appropriate amount of Mg contributing to corrosion protection may be supplied to the plating surface. This effect has been confirmed by stable corrosion resistance in a wet-dry cyclic environment.
The following Patent Literature 4 discloses an invention concerning a technology, by which water or an aqueous solution is sprayed in a form of droplets over the whole area of an unsolidified plated layer from the initiation of solidification of Zn—A—Mg plating until the end of the same in order to improve uneven appearance caused by intermingled crystallization of a MgZn2 phase and a Mg2Zn11 phase during solidification.
This invention alleges that even appearance is obtained owing to presence of a metal structure of an Al primary crystal or a mixture of an Al primary crystal and a Zn single phase in a base of an Al/Zn/Mg2Zn11 ternary eutectic structure owing to the spray cooling.
The following Patent Literature 5 discloses a hot-dip plated steel sheet having excellent appearance characterized in that 60% or more of crystals with a ternary eutectic structure of an Al/Zn/MgZn alloy in a plated layer per unit area have an equivalent circle diameter of 100 μm or more.
This invention alleges that defective appearance may be prevented by suppressing formation of a Mg2Zn11 phase by controlling a cooling condition so as to minimize a supercooled part.
Further, various disclosures have been made also by the following Patent Literature 6 to 12 with respect to the structure of a plated layer of a Zn—A—Mg coated steel sheet.
Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No. 2001-20050
Patent Literature 2: JP-A No. 2003-147500
Patent Literature 3: JP-A No. 2010-100897
Patent Literature 4: JP-A No. H10-265926
Patent Literature 5: JP-A No. 2006-283155
Patent Literature 6: International Publication No. WO 2007/108496
Patent Literature 7: JP-A No. 2004-68075
Patent Literature 8: JP-A No. H10-265926
Patent Literature 9: JP-A No. H10-226865
Patent Literature 10: JP-A No. 2002-047549
Patent Literature 11: JP-A No. 2002-047548
Patent Literature 12: JP-A No. 2002-030405